1. Field of the Invention
The present invention relates to an interface circuit. More particularly, it relates to an interface circuit such as a level conversion circuit adapted for connection to a following circuit which is constituted using metal-semiconductor (MES) type transistors, e.g., field-effect transistors (FETs) formed by compound semiconductors such as gallium arsenide (GaAs).
2. Description of the Related Art
In recent years, integrated circuits using compound semiconductors such as GaAs have been widely used, with an improvement in the degree of integration thereof, in fields where high speed operation and low power dissipation are required. In this case, such a compound semiconductor integrated circuit must be matched with a typical integrated circuit such as a transistor-transistor-logic (TTL) integrated circuit which has been conventionally used. To this end, in a GaAs integrated circuit or the like, it is necessary to provide at an input interface thereof a level conversion circuit for converting an input signal (in this case, TTL level input signal) to the logic level of an internal circuit therein.
For example, as the prior art related to such a level conversion, a technique of converting a TTL level input signal into a complementary metal oxide semiconductor (CMOS) level signal is known. In this case, in an input interface portion of the CMOS integrated circuit, there is provided a level conversion circuit for converting an input signal into a CMOS level signal needed for an internal circuit therein.
Such a CMOS integrated circuit is typically formed via an insulator such as silicon dioxide (SiO.sub.2) on a monolithic semiconductor substrate such as silicon (Si). Accordingly, the structure of each transistor formed on the semiconductor substrate is of a MOS type FET.
On the other hand, in the state of the art, in a compound semiconductor integrated circuit such as a GaAs integrated circuit, an insulator such as SiO.sub.2 as seen in a general CMOS integrated circuit is not put to practical use. Accordingly, the structure of each transistor formed on the compound semiconductor substrate is of a MES type FET.
Namely, in CMOS integrated circuits, there is a gate-insulating film between the gate electrode of each FET and a corresponding channel region in the substrate. As a result, no current flows between the gate electrode and a corresponding source (or drain) region in the substrate. Contrary to this, in compound semiconductor (e.g., GaAs) integrated circuits, each FET has a structure where its gate electrode is Schottky-contacted with the compound semiconductor. As a result, a current path is formed between the gate electrode and a corresponding source (or drain) region in the substrate.
According to the above state of the art, a problem occurs in the case where a preceding circuit is constituted using a current-drive type transistor such as a bipolar transistor and a following circuit is constituted using a MES type FET. Namely, as described above, a current path is formed between the gate and the source (or drain) of the MES type FET in the following circuit. In this state, when the MES type FET is directly connected to an output stage of the preceding circuit, a problem occurs in that a current to be treated as a logic signal in the preceding circuit is absorbed by the current path formed in the following circuit all the time. In other words, a current leak occurs, and thus a problem occurs in that it is impossible for the preceding circuit to normally drive the following circuit.
In this case, if the following circuit is constituted using a MOS type FET, the above problem would not occur since no current path is formed by presence of the above gate-insulating film.